How Battery Problems Start
Your hybrid vehicle’s battery pack contains anywhere from 28 to 40 nickel metal hydride (NiMH) modules assembled in a series. These modules each contain six individual 1.2-volt cells (about the size of a D flashlight battery) made of the same material, also assembled in a series.
Since your car uses these modules to assist power production, their depth of discharge is shallow—less than 10 percent. That causes the unused electrode material to develop a resistive barrier. That reduces its amp-hour capacity. Called the memory effect, it causes the modules to become out of balance with the others.
NiMH batteries discharge more when not in use. After about a week of not being used, a fully charged NiMH battery will have lost about 20 percent of its charge.